The linkage map of MDV has been nearly finished. The linkage map of attenuated strain of MDV and HVT will be constructed to compare with that of MDV. This will reveal the difference of viral DNA structure which may contribute to the oncogenicity and vaccination of these avian herpes viruses. The detailed structural study of homologous region between MDV and HVT will be done by cloning, hybridization, Tm analysis, in vitro translation by hybrid arrested RNA and DNA sequencing. The transcriptional control of viral gene in primary tumors, transplantable tumors, transformed cells and infected cells will be conducted by Southern blot hybridization to identify the transcribed fragment of viral DNA, and by RNA blot hybridization to determine the size of mRNA, and by the Berk and Sharp technique to analyze the splicing. Temperature sensitive mutants will be continuously isolated and characterized by complementation and recombination. Isolation of cold sensitive mutants will also be attempted. These mutants will be useful for establishing transformed cells induced by inoculation of birds (41 C) with cold sensitive mutants and analyzing the function of viral gene in shift up and shift down experiments.